Rheological and Optical Properties of Shearing Colloidal Suspensions by Polarized Light Spectroscopy
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RHEOLOGICAL AND OPTICAL PROPERTIES OF SHEARING COLLOIDAL SUSPENSIONS BY POLARIZED LIGHT SPECTROSCOPY NORMAN J. WAGNER Department of Chemical Engineering, University of Delaware Newark, DE 19716 ABSTRACT The two-particle correlation function serves as an important connection between suspension microstructure, scattering experiments, and macroscopic properties. Formal relationships between the experimentally accessible structure factor for suspensions undergoing simple shear, bulk stresses, and flow dichroism and birefringence are demonstrated. Measurements of flow dichroism using variable wavelength incident radiation provide a method to map out the nonequilibrium microstructure. INTRODUCTION Theoretical treatments of complex fluids from molecular or micromechanics generally start from a one component fluid model (OCF) where the suspending fluid is treated as a continuum fluid, entering only as a parameter in the potential of mean force between the interacting macroparticles. The configurational part of the partition function, namely the microstructure, then serves to define the state of the system once these forces are known. All of the bulk macroscopic properties of the complex fluid are then formally written as ensemble averages over this microstructure, in both equilibrium and nonequilibrium conditions. Hence, the central importance of the microstructure in determining bulk mechanical and optical properties of the suspension. When subject to deforming mechanical perturbations, that are easily accessible at standard laboratory or processing conditions, the resulting nonequilibrium microstructure deformation manifests itself in rheological responses such as elasticity, normal stress differences, and shear thinning. These nonequilibrium structures, calculated from balancing the shear deformation of the microstructure against the many-body thermodynamic and hydrodynamic forces acting between the particles, also result in measurable changes in the scattering properties of the suspension, as measured by light, X-ray, or neutron scattering techniques [1, 2, 3, 4, 5]. As there has been a significant effort to both calculate these nonequilibrium microstructures and the resulting rheological response for model systems [6, 7, 8, 9, 10, 11, 12, 131, it is of interest to develop experimental techniques that can directly measure the nonequilibrium microstructure of shearing model suspensions for direct tests of the theories. Further, as determination of the microstructure and its anisotropy is important in determining the mechanical response of both the fluid and products made from the fluid, such a technique may be important in the materials processing industry. Polarized light spectroscopy measures the transmission of light through a medium with anisotropic refractive index. Any anisotropic nonequilibrium microstructure in a complex fluid will result in an anisotropic refractive index for the fluid, and is therefore measurable by this technique. Reported here are results for the techniques of flow dichroism and flow birefri
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